Chronic obstructive pulmonary disease (COPD) and obstructive sleep apnea (OSA) were common disorder that has been gaining increased attention as an important factor affecting health. The coexistence of COPD and OSA, which was termed overlap syndrome, is assumed to have affected many patients. The increases in morbidity or mortality are higher in patients with overlap syndrome than in patients with OSA or COPD alone. Nevertheless, the impact of OSA on COPD is not yet known. Patients with COPD experience sleep-related abnormalities, including sleep-disordered breathing associated with low sleep quality and hypoxia. Continuous positive airway pressure treatment accompanied by oxygen therapy has recently been attracting attention as the treatment of choice for overlap syndrome. Clinical consideration should be given to its potential sleep-related features that have not yet been recognized in order to improve clinical symptoms and quality of life for patients affected by overlap syndrome.